Making castings of aluminum and aluminum-base alloy



Patented June 6, 1939 UNITED STATES PATENT OFFICE MAKING CASTINGSALUMHVUM- John Herbert Alden New Kensington,

OF ALUMINUM AND BASE ALLOY .1 Philip Trimble Stroup, and Paaqual MichaelBudge, Fairfield, Conn, assignors to Aluminum Company of America, rationof Pennsylvanl Pittsburgh, Pa., a corpoa No Drawing. Application March9. 1938.

Serial No. 194,766

13 Claims.

This invention relates to the casting of aluminum and aluminum-basealloys, that is, alloys in which aluminum is not less than about 50 percent of the whole, particularly aluminum alloys containing copper as theprincipal alloying element or one of such elements. More specificallythe invention relates to the prevention of the defect commonly known as"pinhole porosity" in castings, especially castings of intricate shape.

This defect has long been known in the art, and

various methods of combating it have been suggested. based 'on thetheory that it is caused by the presence of gas in the melt and therelease of the gas as the metal solidifies. One method proposed forovercoming the evil consists in mechanically agitating the molten metalin an atmosphere of nitrogen, carbon monoxide, carbon dioxide, or otherhydrogen-free gas which is also free from water vapor, while the surfaceof the melt is covered with a flux capable of dissolving oxides presentor formed during the agitation. This process is based on the theory thatthe most important factor is the taking up of hydrogen by the melt whichfrequently comes from the reduction of water by the hot metal insuflicient amount to impair to a serious extent the de-gassing effectedby the mechanical agitation. Mechanical agitation, however, isinadequate in some cases, as in casting finned cylinder heads foraircraft engines. In such cases our experience shows that to insuresubstantially complete freedom from pinhole porosity the agitationshould be eifected by bubbling a gas through the molten metal whileprotecting the metal from contact with excessive moisture. We havefurther found that the use of an atmosphere of nitrogen or an oxide ofcarbon is unnecessary, and that an atmosphere of dried air, otherwise ofnormal constitution, is equally if not more effective. The essentialfeature of our process accordingly consists in bubbling a gas throughthe molten metal while the metal is in an atmosphere of dry air. Ingeneral the moisture-content of the atmosphere should not exceed 1 grainper cubic foot. and may with ad- 46 vantage be not more than .5 grainper cubic foot,

especially when the alloy contains copper in substantial amount, sayabout 2 per cent or more. The permissible moisture content of the airdepends chiefly upon the pouring temperature and the qualitative andquantitative composition of the metal, the pouring temperature itselfdepending largely upon the intricacy of the casting. These factors arewidely variable, and it will therefore be recognized that an upper limitof as 1 grain per cubic foot is to a certain extent arbitrary. That is,it is not critical as meaning that more moisture is always fatal and alittle less always effective, but experience indicates that in generalmore than the amountjust stated is apt to be ineffective, and that forcastings having many thin sections and thus requiring a relatively highpouring temperature the moisture content should not be more than .5grain per cubic foot. The bubbling oi the gas through the molten metalhas a fluxing effect in that dross inclusions and the like are broughtto the surface, and if the passing of the gas through the melt iseffected in an atmosphere of dried air and continued in such atmosphereuntil the metal is ade-' quately clean it will usually be found thatenough gas has been removed to permit castings to be made which aresubstantially free from the defects sought to be eliminated, as can bedetermined by trial.

I'he de-gassing of the molten alloy can be eifected to any extentnecessary or desirable by means of any hydrogen-free non-oxide-forminggas, preferably dry chlorine or other halogen. By hydrogen-free gas wemean one which does not contain or release hydrogen at the temperatureof the de-gassing operation. The gas can be introduced as such throughthe bottom or sides of the vessel containing the melt, or through one ormore pipes extending into it, or by the thermal decomposition orvolatilization of one or more compounds, by preference ahalogen-containing compound, submerged in the molten metal. Aluminum orzinc chloride can be used advantageously, or a mixture of the two. Otherconditions being the same, the extent of de-gassing necessary in ourprocess for the production of castings substantially free from pinholeporosity depends largely upon the qualitative and quantitativecomposition of the metal. For an alloy composed of copper 10 per cent,iron 1 per cent, magnesium .25 per cent, and the rest aluminum with theusual impurities, an effective practice is to pass through the melt suchquantity of chlorine as would amount (at atmospheric pressure) to 0.03to 0.04 cubic foot per pound of metal. A convenient method for thepurpose is to draw oil from a cylinder of the gas at high pressure anamount of gas which would, when expanded to a convenient lower pressure,afford the above quantity per pound of metal. For example, for a melt of2'75 pounds of the above alloy having an exposed surface area of about300 square inches and a depth of about 13 inches, we are accustomed totake about 3.15 cubic feet of chlorine at 60 pounds pressure per squareinch, discharge the gas from perforations around the lower portion of apipe extending into the melt from the top down to the bottom of thevessel, and continue the discharge until the pressure (as shown by thegauge on the cylinder containing the gas) has fallen to 10 pounds. Thisusually takes about 12 minutes. We have also found that the amountstated is suitable for other compositions, for example an alloy composedof copper 4 per cent, nickel 2 per cent, magnesium 1.5 per cent, and therest aluminum with the usual iron and silicon impurities. In any case anadvantageous amount can be determined by trial with a few castings. Wehave observed further that with different batches of alloy of supposedlythe same composition it is sometimes necessary to increase the amount ofchlorine, and that on the other hand the amount may sometimes bedecreased. Alloys containing still less copper, and even none at all,are in some cases liable to give castings having pinhole porosity, andwhere this type of porosity is found to occur with such alloys they willin general be benefited by the treatment described above, and in somecases, easily determined by trial, the amount of chlorine can bedecreased.

The following is an example of practicing our invention in an efficientmanner. The alloy, previously compounded to a desired composition, ismelted and skimmed in the ordinary way, usually without fiuxing, and istransferred to a holding furnace through which a current of dried air ispassed. The drying of the air may be efi'ected in any convenient way,preferably by! means of activated alumina or other eiilcient adsorbentof moisture. The holding furnace need not be closed if the current ofdried air is of sufficient volume to insure protection of the metal fromoutside air which contains more than the minimum amount of moisture asdetermined by experience with the particular alloy and other conditionsobtaining. While in the atmosphere of dehydrated air the melt isde-gassed by bubbling chlorine through it. and the metal is raised tothe desired pouring temperature. In general we have found itadvantageous to maintain a dry air atmosphere over the melt until themetal is ladled out for pouring into the molds. Notwithstanding that thedry air contains about 20 per cent of oxygen, and that the hot,agitated, readily oxidizable metal is therefore constantly bathed or incontact with a large volume of free oxygen, we have found that theresulting oxidation is inconsequential. This we have observed to be truein the difficult operation of casting airplane engine cylinder heads,elaborately finned for cooling purposes, from an aluminum alloycontaining copper 10 per cent, iron 1 per cent, and a small amount ofmagnesium (less than 1 per cent. say .25 per cent).

In this case the melt was raised, in the holding furnace, from atemperature between 1200" and 1250 F. to a pouring temperature between1400 and 1500 F., with the current of dry air (containing a residualmoisture content of about .5 grain per cubic foot) maintained at fullvolume, which for about 2'75 pounds of metal, having an exposed area ofabout 300 square inches and a depth of approximately 13 inches, ispreferably 135 to 140 cubic feet per minute; though for purposes ofeconomy about a quarter of the volume stated is employed in completingthe heating of the melt to the pouring temperature if the desiredtemperature has not been attained during the degassing operation.

In another emcient practice of our method,

with the 4 per cent copper alloy given above, the melting, de-gassing,and raising to the pouring temperature were effected in the samefurnace. With 300 pounds of the alloy in as oylindrical vessel 1'!inches, approximately, in diameter inside, and having a hemisphericalbottom, the exposed surface area of the melt is about 227 square inchesand the greatest depth is about 20 inches. With this charge the driedair was turned on when the metal began to melt, at which time thetemperature is about 1100 F. Maintaining the fire under the pot thetemperature is brought up to 1400" F. The passing of chlorine throughthe melt is then begun, approximately 16 or 1'? cubic feet of the gasbeing passed through in about 18 minutes. During this operation thetemperature of the melt may fall somewhat, say to 1350 F. A temperatureof 1380 F. is suitable for this alloy but to allow for incidentalcooling between the furnace and the mold the melt is brought to 1400" F.or somewhat higher, the flow of dried air over the metal being continuedduring this reheating. In the degassing operation the chlorine used wastaken from a tank containing approximately 3.5 cubic feet of the gas at60 pounds pressure per square inch above atmospheric, and the gas wasbubbled through the molten metal until the pressure had fallen almost toatmospheric.

It is to be understood that the invention is not limited to the detailsherein specifically described but can be carried out in other wayswithout departing from its spirit as defined by the subjoined claims.

We claim-- 1. In a method of making castings oi aluminum andaluminum-base alloys free from pinhole porosity, the step comprisingcle-gassing the molten metal by bubbling a non-oxidizing hydrogen-freegas through the metal while in an atmosphere of air in which any contentof moisture present has been reduced to not more than 1 grain per cubicfoot.

2. In a method of making castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, protecting the moltenalloy from contact with excess moisture by means of an atmosphere of airin which the moisture content is not more than 1 grain per cubic foot,and while the alloy is so protected de-gassing it by bubbling chlorinethrough the molten metal.

3. In a method of making castings of aluminum and aluminum-base alloysfree from pinhole porosity, comprising melting the alloy. Drotecting themolten alloy from contact with excessive moisture by means of anatmosphere of air in which the moisture content is not more than .5grain per cubic foot, and while the alloy is so protected, de-gassing itby bubbling a nonoxide-forming gas through the molten metal, and heatingthe metal to the pouring temperature.

4. In a method of making castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, transferring the alloy toa holding chamber, drying air to a moisture content not more than '1grain per cubic foot, delivering such dried air to the holding chamberto provide a protective atmosphere therein, and while the alloy is insuch atmosphere de-gassing it by bubbling a non-oxidizing hydrogen-freegas through the molten metal.

5. In a method of making castings of aluminum and aluminum-base alloysfree from pin- 15 hole porosity, melting the alloy, transferring thealloy to a. holding chamber, providing therein an atmosphere of air inwhich the moisture content is not more than .5 grain per cubic foot, andwhile the alloy is in such atmosphere de-gassing it by bubbling anon-oxidizing hydrogen-free gas through the molten metal.

6. In a method oi making castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, transferring the alloy toa holding chamber, providing therein an atmosphere in which the moisturecontent is not more than .5 grain per cubic foot, and while the alloy isin such atmosphere de-gassing it by bubbling chlorine through the moltenmetal.

'7. In a method 01' making castings o1 aluminum-copper alloy free frompinhole porosity, the step comprising de-gassing the molten metal bybubbling a non-oxidizing hydrogen-free gas through the metal while in anatmosphere of air from which any content of moisture present has beenreduced to not more than .5 grain per cubic i'oot.

8. In a, method of making castings oi aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy in an atmosphere oi! airin which any content of hoisture has been reduced to not more than 1grain per cubic foot, and while in such atmosphere de-gassing the alloyby bubbling chlorine through it.

9. In a method of maldng castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the allow in a current of air havinga moisture content not exceeding .5 grain per cubic foot, de-gassing thealloy by bubbling through it a non-oxidizing hydrogenfree gas while thealloy is protected by such current, and thereafter decreasing the volumeof the current and while the alloy is protected by the current ofdecreased volume raising the alloy to a desired pouring temperature.

10. In a method or making castings of aluminum and aluminum-base alloystree from pinhole porosity, de-gassing the molten alloy in a current ofair having a moisture content not exceeding .5 grain per cubic foot.

11. In a method of making castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, transferring the alloy toa holding chamber, passing through the chamber a current of air having amoisture content not exceeding .5 grain per cubic foot, degassing thealloy by bubbling through it a nonoxidizing hydrogen-free gas while thealloy is protected by such current, and thereafter decreasing the volumeof the current and while the alloy is protected by the current ofdecreased volume raising the alloy to a desired pouring temperature.

12. In a method of making castings oi aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, transferring the alloy toa holding chamber, passing through the chamber a current of air having amoisture content not exceeding .5 grain per cubic foot, degassing thealloy by bubbling chlorine through it while it is protected by suchcurrent, and thereafter decreasing the volume of the current and whilethe alloy is protected by the current of decreased volume raising thealloy to a desired pouring temperature.

13. In a method of making castings of aluminum and aluminum-base alloysfree from pinhole porosity, de-gassing the molten alloy in a current orair having a moisture content not exceeding .5 grain per cubic foot bybubbling chlorine through the alloy, and thereafter decreasing thevolume of the current and while the alloy is protected by the current ofdecreased volume raising the alloy to a desired pouring temperature.

JOHN HERBERT ALDEN. PHILIP 'I'RIMBLE STROUP. PASQUAL MICHAEL BUDGE.

CERTIFICATE OF CORRECTION.

Patent no. 2,160,812.

June a 9 JOHN HERBERT ALDEN, ET AL.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 3,first column, line 27, claim 8, for "hoisture' read moisture; line 35,claim 9, for the word "allow" read alloy; and that the said LettersPatent should be read of the case in the Patent Office.

with this correction thereinthat the some may conformto the recordSigned and sealed thiellth day of July, A. D. 1939.

(Seall Henry Van Arsdale Acting Commissioner oi Patmts.

hole porosity, melting the alloy, transferring the alloy to a. holdingchamber, providing therein an atmosphere of air in which the moisturecontent is not more than .5 grain per cubic foot, and while the alloy isin such atmosphere de-gassing it by bubbling a non-oxidizinghydrogen-free gas through the molten metal.

6. In a method oi making castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, transferring the alloy toa holding chamber, providing therein an atmosphere in which the moisturecontent is not more than .5 grain per cubic foot, and while the alloy isin such atmosphere de-gassing it by bubbling chlorine through the moltenmetal.

'7. In a method 01' making castings o1 aluminum-copper alloy free frompinhole porosity, the step comprising de-gassing the molten metal bybubbling a non-oxidizing hydrogen-free gas through the metal while in anatmosphere of air from which any content of moisture present has beenreduced to not more than .5 grain per cubic i'oot.

8. In a, method of making castings oi aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy in an atmosphere oi! airin which any content of hoisture has been reduced to not more than 1grain per cubic foot, and while in such atmosphere de-gassing the alloyby bubbling chlorine through it.

9. In a method of maldng castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the allow in a current of air havinga moisture content not exceeding .5 grain per cubic foot, de-gassing thealloy by bubbling through it a non-oxidizing hydrogenfree gas while thealloy is protected by such current, and thereafter decreasing the volumeof the current and while the alloy is protected by the current ofdecreased volume raising the alloy to a desired pouring temperature.

10. In a method or making castings of aluminum and aluminum-base alloystree from pinhole porosity, de-gassing the molten alloy in a current ofair having a moisture content not exceeding .5 grain per cubic foot.

11. In a method of making castings of aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, transferring the alloy toa holding chamber, passing through the chamber a current of air having amoisture content not exceeding .5 grain per cubic foot, degassing thealloy by bubbling through it a nonoxidizing hydrogen-free gas while thealloy is protected by such current, and thereafter decreasing the volumeof the current and while the alloy is protected by the current ofdecreased volume raising the alloy to a desired pouring temperature.

12. In a method of making castings oi aluminum and aluminum-base alloysfree from pinhole porosity, melting the alloy, transferring the alloy toa holding chamber, passing through the chamber a current of air having amoisture content not exceeding .5 grain per cubic foot, degassing thealloy by bubbling chlorine through it while it is protected by suchcurrent, and thereafter decreasing the volume of the current and whilethe alloy is protected by the current of decreased volume raising thealloy to a desired pouring temperature.

13. In a method of making castings of aluminum and aluminum-base alloysfree from pinhole porosity, de-gassing the molten alloy in a current orair having a moisture content not exceeding .5 grain per cubic foot bybubbling chlorine through the alloy, and thereafter decreasing thevolume of the current and while the alloy is protected by the current ofdecreased volume raising the alloy to a desired pouring temperature.

JOHN HERBERT ALDEN. PHILIP 'I'RIMBLE STROUP. PASQUAL MICHAEL BUDGE.

CERTIFICATE OF CORRECTION.

Patent no. 2,160,812.

June a 9 JOHN :m' :m- ALDEN, ET AL.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 3,first column, line 27, claim 8, for "hoisture' read moisture; line 35,claim 9, for the word "allow" read alloy; and that the said LettersPatent should be read of the case in the Patent Office.

with this correction thereinthat the some may conformto the recordSigned and sealed thiellth day of July, A. D. 1939.

(Seall Henry Van Arsdale Acting Commissioner oi Patmts.

